This invention pertains generally to protective surface coatings commonly known as ;paint coatings which are organic compositions applied to substrates to form continuous films and are cured or otherwise hardened to provide protection as well as a decorative appearance to the substrates. Protective surface coatings ordinarily comprise an organic polymeric binder, pigments, inert fillers and other paint coating additives. The polymeric binder functions as an organic vehicle for the pigments, inerts, and other additives and further functions as a binder for the pigments and inert fillers in the cured or hardened paint film. Polymeric binders can be thermosetting binders based on coreactive binder components.
Powder coatings are organic coatings in dry powder form comprising essentially 100% nonvolatile solids based on a polymeric film forming binder. Powder coatings can be clear coatings without opacifying pigments or pigmented coatings containing opacifying pigments. Powder paints are finely pulverized polymeric compositions in the form of a dry free-flowing fine powder which melts and flows on a substrate at elevated temperatures to produce a smooth coated substrate surface. Protective surface coatings based on powder paints are particularly useful in view of environmental constraints in that powder paints ordinarily do not contain organic solvents or emit organic compounds upon baking or heat curing.
Thermosetting powder coatings ordinarily are heat thermosetting coatings and can be based on blocked isocyanate crosslinkers such as disclosed in U.S. Pat. No. 3,822,240 and U.S. Pat. No. 3,931,117. In this regard, solid blocked isocyanates are commonly the reaction product of a polyol such as ethylene glycol or trimethylol propane with an isocyanate and a blocking agent such as caprolactam or MEK oxime. The function of the polyol and the amount used is to raise the softening point of the crosslinker whereby the physical blend will be stable and the resultant powder coatings will not sinter or fuse into lumps at room temperature or at storage temperatures up to 45.degree. C.
Since isocyanates are expensive, the lowest cost blocked isocyanate would be a simple blocked isocyanate such as caprolactam blocked IPDI, but this is not used despite the low equivalent weight because the softening point is too low and the crosslinker would only be difunctional. The softening point of the crosslinker cannot readily be compensated for by raising the Tg of the resin, especially if the polymer is a polyester, without having to raise the molecular weight and/or use expensive raw materials, or both. If the functionality of the polyester is increased to compensate for the lower functionality of a difunctional crosslinker, the hydroxyl number will be increased for a given molecular weight thus increasing the amount of crosslinker required. Another approach to making blocked isocyanates with a better tack temperature at lower equivalent weight is by trimerization of part of the isocyanate, which is described in the following patents: U.S. Pat. Nos. 4,252,923; 4,302,351; and 4,354,014. These processes/compositions are based on blocked isocyanate trimers which are inherently more expensive than the compositions of this invention.
It now has been found that substantially improved thermosetting powder paints can be produced by crosslinking hydroxyl functional polymers upon heating with a blocked isocyanate comprising a pentaerythritol isocyanate having unreacted isocyanates blocked with a blocking agent such as e-caprolactam. Use of pentaerythritol as the major polyol component by reacting with excess equivalents of isocyanate and producing a blocked isocyanate crosslinker provides substantially improved powder coatings exhibiting higher tack temperatures at a given equivalent weight as well as lower melt viscosities. It has been found that major amounts of pentaerythritol can be used in conjunction with lesser amounts of aliphatic diols Such as 1,4-butane diol, 1,6-hexane diol to yield blocked isocyanate to give improved impact with acrylic polymers.
Although U.S. Pat. Nos. 3,822,240 and 3,931,117 describe the manufacture of blocked isocyanates and uses in crosslinking hydroxyl functional polyesters and acrylic resins in powder coatings, pentaerythritol is not mentioned as a polyol to make the blocked isocyanate.
Advantages of the present invention include lower raw material costs due to ability to produce the higher tack temperatures at lower equivalent weight as well as the ability to use most any type of isocyanates which do not trimerize such as Desmodur W (dicyclohexylmethane 4,4'diisocyanate); m-TMXDI benzene, 1,3 bis(isocyanate, 1 methyl-ethyl). The preferred method of synthesis is to charge the molten caprolactam, add the pentaerythritol at about 130.degree. C., and dissolve the pentaerythritol in the caprolactam. The isocyanate is then added and the exotherm allowed to raise the batch temperature to about 140.degree. C. and the temperature controlled by the rate of addition of the isocyanate.
The process of this invention is contrasted with the use of liquid or lower melting polyols conventionally used where a narrow molecular weight distribution is achieved by melting the polyol and add to the pre-reacted mixture of caprolactam and isocyanate. Accordingly, this invention provides an improved solid blocked isocyanate crosslinker for powder coatings using pentaerythritol as the branching agent for the isocyanate blocked crosslinking agent. By using pentaerythritol rather than the more commonly used polyols, solid blocked isocyanate crosslinkers can be made which are better suited for use as crosslinkers in powder coatings due to higher tack temperature at a given equivalent weight and lower melt viscosities. These and other advantages will become more apparent by referring to the detailed description and illustrative examples.